Orientation
Star-gazing requires some basic
directions and terminology, as well as a system for measuring time. This
section also introduces the charts astronomers use to represent the entire
night sky.
Background
Astronomy would be pretty easy, and
pretty dull, if the Earth didn't move. In fact, the Earth has several different
motions which must be understood in order to do astronomy. One of these motions
is the Earth's rotation on its axis; another is the Earth's revolution,
or orbital motion, about the Sun.
ROTATION
If you watch the night sky for a few
hours, you will see that the stars appear to rotate about a fixed point in the
sky (which happens to be near the pole star, Polaris). This motion is due to
the Earth's rotation. As the spin of the Earth carries us eastward at almost
one thousand miles per hour, we see stars rising in the East, passing overhead,
and setting in the West. The Sun, Moon, and planets appear to move across the
sky much like the stars.
Because of the Earth's rotation, everything
in the sky seems to move together, turning once around us every 24 hours.
Ancient astronomers explained this phenomenon by supposing that the Sun, Moon,
planets, and stars were attached to a huge celestial sphere,
centered on the Earth, which rotated on a fixed axis once per day. Of course,
this sphere does not really exist; the Sun, Moon, planets, and stars all
fall freely through space, and only appear to move together because of
the Earth's rotation. Nonetheless, we still use the concept of the celestial
sphere in talking about the positions of stars.
The celestial sphere appears to rotate
around a fixed point, the North celestial pole, which is 21.3°
above the horizon as seen from
Since the apparent rotation of the
celestial sphere is due to the actual rotation of the Earth, the North
celestial pole is exactly overhead as seen from Earth's North Pole. Likewise,
every point on the celestial equator is exactly overhead from
some point on the Earth's equator.
REVOLUTION
Over the course of a year, the Earth
makes one complete orbit about the Sun. As a result, the Sun seems to
move with respect to the stars, appearing in front of one constellation after
another, as shown in the diagram on p. 12 of Stars & Planets.
After one year, the Sun is back where it started. The Sun's annual path across
the sky is called the ecliptic. Traditionally, the ecliptic was
divided into twelve equal parts, each associated with a different constellation
of the zodiac.
The night-time sky is just the part of
the sky that we see when the islands of
The Earth's axis of rotation is not precisely
parallel to its axis of revolution; the angle between them is 23.5°.
Consequently, the ecliptic is inclined by the same angle of 23.5° with respect
to the celestial equator. This misalignment causes seasons; when the Sun
appears North of the celestial equator the Earth's northern hemisphere receives
more sunlight, while when the Sun appears South of the celestial equator the
northern hemisphere receives less sunlight.
If we could view the Solar System from a
point far above the North Pole, we'd see the Earth rotating counter-clockwise
on its axis and revolving counter-clockwise about the Sun. Most of the other
planets would also appear to rotate counter-clockwise. In addition, the Moon
would appear to orbit the Earth in a counter-clockwise direction, as would most
other planetary satellites.
TIME
In this class, we will use a 24-hour
clock instead of writing `am' or `pm'. Since our class meets in the evening,
most of the times we will record are after noon, and the 24-hour time is the
time on your watch plus 12 hours. For example, our class starts at 19:00 (=
7:00 pm + 12:00), and ends at 22:00 (= 10:00 pm + 12:00). Sometimes we need to
record the date and the time together; for example, our first class begins at
01/14/03, 19:00.
Astronomers all over the world use a
single time system to coordinate their observations. This system is called Universal
Time, abbreviated as UT or UTC. (Greenwich Mean Time, abbreviated GMT, is
the same thing as UT.) Universal Time is exactly 10 hours ahead of
Hawaii time. To convert 24-hour Hawaii time to UT, you add 10 hours; if
the result is more than 24, subtract 24 and go to the next day. For example,
our first observing session (weather permitting) will be at
01/21/03, 19:00, or 01/22/03, 05:00 UT. To
convert from UT to Hawaii time, you
subtract 10 hours; if the result is less than 0, you add 24 and go to the
previous day. For example, Mercury will pass in front of the Sun from
05/07/03, 05:13 UT to 05/07/03, 10:32 UT; that's 05/06/03, 19:13 to 05/07/03, 00:32
in our time zone. (Unfortunately, the Sun will have set here on
As a rule, we will use 24-hour Hawaii time in this class, and write the time without any time
zone. The `UT' symbol will be used only when we want to specify
universal time.
ALL-SKY CHARTS
Astronomers represent the appearance of
the entire sky as seen at some particular place and time by drawing circular all-sky
charts. Unfortunately, it's not really possible to capture the
appearance of the sky on a flat piece of paper, so reading an all-sky chart and
relating it to what you see in the sky is a bit tricky. For example, these
charts distort the patterns of stars near the horizon, so you may find it hard
to recognize constellations from an all-sky chart. The only way to correct this
distortion is to break the sky up into several separate charts (this is the
approach used in The Sky Tonight, which we will use to find the
constellations). For some purposes, however, it's very convenient to
show the entire sky in one chart, so you should learn to read an all-sky chart.
To read an all-sky chart, hold it in
front of you with the side labeled `N' at the top. Now imagine you are
lying flat on your back with your head pointing North;
then East will be on your left, South at your feet, West on your right, and the
Zenith right in front of you. Mentally stretch the disk of the chart so that it
forms a dome over your position. The positions of stars on this imaginary dome
now correspond to their positions in the sky.
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The
sky over |
If you try this exercise with the chart
shown here, you can get a pretty good idea of how the sky will look on
01/21/03, 21:00. For example, Saturn is near the center of chart, so it
will be almost exactly overhead. Jupiter is on the left side of the chart,
about one-third of the distance from the edge to the center, so it will be
visible in the East, about one-third of the way from the horizon to the Zeneth.
The North Star, Polaris, is near the top of the chart, so it will be visible in
the North; the Little Dipper hangs down toward the horizon from Polaris, and it
will only be visible if we have a good view toward the North (which Kapiolani
park, alas, does not).
CELESTIAL COORDINATES
Note: this is an advanced topic. We
won't have much use for celestial coordinates in this class, but you'll see
them mentioned from time to time.
Just as latitude and longitude can be
used to specify any point on the Earth's surface, two celestial
coordinates can be used to specify any point on the celestial sphere.
Imagine starting from the point on the sky the point where the Sun, moving
North, crosses the celestial equator (this is the point labeled `0 h' in
the chart above). To reach any given point on the celestial sphere, you could
first travel along the celestial equator, and then towards one of the celestial
poles, until you reach your destination. The angle you've traveled along the
equator is called the right ascension; it's measured in units of
hours, where 1 hour = 15°. The angle you've traveled towards one of the
poles is called the declination; it's measured in degrees, with
positive declinations towards the North celestial pole, and negative
declinations towards the South celestial pole.
As already noted, celestial coordinates
won't be used much in this class. They're included here because they are used
in Stars & Planets. Typically, the book gives celestial coordinates
when discussing stars; for example, if you look at the description of alpha
Orionis on p. 194, you'll see `5h 55m +7°.4' just after the
star's name. This means that alpha Orionis has a right ascension of 5h 55m
(just slightly less than 6 hours) and a declination of +7°.4. Celestial
coordinates also appear on the constellation charts; for example, see the chart
of Orion on p. 195, which shows that Orion lies across the celestial
equator at about 5h 30m right ascension.
HAWAIIAN COORDINATE SYSTEMS
Being
island dwellers, the Hawaiians developed a land-based system of directions
which you may be familiar with. The
words makai and mauka are used in everyday conversation, but very few people
actually know what they mean. The word
ma in Hawaiian means “in, at, with, the surroundings of, the environs of, etc.” So ma-uka means the environs (ma) of the
uplands (uka). The same is true for ma-kai
(at the sea). These directions are
useless on the ocean if you are farther than about 100 miles from an
island. In that case every direction is
makai. The Hawaiians traveled thousands
of miles on open ocean to find Hawai’i
The point on the horizon where
the sun rose was called kükulu hikina, Kükulu means the direction and hikina
means to rise. The sun set at kükulu
komohana (komo means to enter).
If a
person lays on his back on the ground with his head pointing to the direction
of the sunrise, his left hand will point to the south, kükulu hema, his right
to the north, kükulu akau. His right
side of his body is called akau and his left is hema, matching the sky
directions.
If
he tries to rise, usually his head will rise first (hikina) and if he were to
enter the Earth, he would walk down in the direction of his feet (komohana).
WEB RESOURCES
An interactive
planetarium, set up to show the sky now above
Shows how the
sky above
Shows how the
sky above
REVIEW QUESTIONS
- If you face North, which way does the
celestial sphere appear to rotate - clockwise or counter-clockwise?
- If you see the full Moon rising in the East in the evening,
where would you expect to see it very early next morning?
- What is 01/20/03, 7:35 UT in
local 24-hour time? What day of the week? Is it morning or evening?
- What direction would you face to see the red `x' in the all-sky
chart above? How far above the horizon should you look?